Printing plates suitable for offset lithographic printing are known which comprise a support having a surface having hydrophilic non-image areas and hydrophobic ink-receptive image areas. The art of lithographic printing is based upon the immiscibility of oil and water, wherein oily ink is preferentially retained by the image area and water or fountain solution is preferentially retained by non-image areas.
Ink jet printing mechanisms can be categorized as either continuous ink jet or drop-on-demand ink jet. Graphic arts require small dots for acceptable resolution, and drop-on-demand ink jet printers are at present incapable of producing drops sufficiently small for the graphic arts industry. Continuous ink jet printing, which can produce acceptably small drops, dates back to at least 1929. See U.S. Pat. No. 1,941,001 to Hansell.
U.S. Pat. No. 4,833,486 describes an apparatus for the production of lithographic printing plates by melting a solid hydrophobic ink and spraying droplets of the melted ink onto the plate in a pattern corresponding to the image with an ink jet head.
U.S. Pat. No. 5,501,150 describes a process for the manufacture of a lithographic printing plate by image-wise projection of droplets of liquid onto a hydrophilic surface thus to bring together a reducible silver compound, a reducing agent for the silver compound, and physical development nuclei to catalyze the reduction of the silver compound to silver metal. The silver image is hydrophobized to produce the printing plate.
U.S. Pat. No. 4,303,924 describes a ink jet printing process utilizing a radiation curable ink composition comprising an ethylenically unsaturated material, a reactive synergist, a dye colorant and an oil soluble salt for conductivity.
U.S. Pat. No. 5,511,477 claims the method of preparing photopolymer relief type printing plates such as flexographic printing plates by use of a radiation cured photopolymeric ink containing ferromagnetic powder for conductivity.
U.S. Pat. No. 5,495,803 discloses a method of forming a graphic arts photomask, also known as a color separation image, by ink jet printing. The photomask is then used to expose a sensitized lithographic printing plate.
JP-A-53015905 discloses the preparation of a printing plate by ink-jetting an alcohol-soluble resin in an organic solvent onto an aluminum plate.
JP-A-56105960 describes the formation of a printing plate by ink-jetting an ink capable of forming an oleophilic surface, which ink contains a hardening substance such as an epoxide or photo-hardening substance onto a hydrophilic substrate.
EP-776763-A1 describes a method for producing a lithographic printing plate by ink jet printing of two reactive inks which combine to form a polymeric resin which forms the ink accepting part of the lithographic printing plate image.
Kokai 62-25081 describes a lithographic printing plate made by ink-jetting an oleophilic liquid onto an aluminum plate.
WO 94/1191 describes in ink jet apparatus for the production of lithographic printing plates.
U.S. Pat. No. 4,599,627 describes an apparatus for ink jet printing which jets two different inks onto the same spot, using electrostatic deflection of conducting inks to control the printing.
U.S. Pat. No. 5,466,658 describes a method for preparing relief images (flexographic printing plates) by ink-jetting a reagent that reacts with a polymer on a substrate to render it insoluble in the subsequent processing solution.
U.S. Pat. No. 5,168,288 describes a laser thermal color proofing system, wherein absorption of laser light heats a dye laser thus causing the dye to be transferred to a receiver. Such a system, while effective for producing proofs, is expensive because of the cost of preparing donor sheets.
U.S. Pat. No. 5,492,046 describes a method for preparing a lithographic printing plate by laser thermal exposure of the sensitized plate followed by solution processing. While effective, such systems are expensive because of the high cost of the multiple high power lasers that are required.
Inks for high speed ink jet printers must have a number of special characteristics. Typically, water-based inks have been used because of their conductivity and viscosity range. Thus, for use in a conventional ink jet printer the ink must be electrically conductive, having a resistivity below about 5000 ohm-cm and preferably below about 500 ohm-cm. For good runability through small orifices water-based inks generally have a viscosity in the range between about one and fifteen centipoise at 25.degree. C.
Additionally, the ink must be stable over a long period of time, compatible with the materials comprising the orifice plate and ink manifold, free of living organisms, and functional after printing. The required functional characteristics after printing are: smear resistance after printing, fast drying on paper, and waterproof when dry. Examples of different types of water-based ink jet printing inks are found in U.S. Pat. No. 3,903,034; No. 3,889,269; No. 3,870,528; No. 3,846,141; No. 3,776,642; and No. 3,705,043.
Water-based inks in general can be said to have the following problems:
(1) They require a large amount of energy to dry after printing. PA1 (2) Large printed areas on paper usually cockle because of the amount of water present. PA1 (3) The printed images are sensitive to wet and dry rub. PA1 (4) The compositions of the ink usually require an anti-bacterial preservative to minimize the growth of bacteria in the ink. PA1 (5) The inks tend to dry out on the tip of the orifice resulting in clogging.
Some of these problems may be overcome by the use of polar, conductive organic solvent based ink formulations. However, the use of non-polar organic solvents is generally precluded by their lack of electrical conductivity.
Scitex has demonstrated, at the Print '97 show in Chicago, an Iris jet drop printer with a special liquid writing on grained, anodized aluminum plates to make an offset lithographic printing plate. They showed the press run from such plates. However, the Iris jet drop printer is slow, because it has a limited number of nozzles. Adding more nozzles is expensive. In addition, the use of the electrostatic deflection system in the Iris jet drop printer limits the kinds of liquids that can be used.
U.S. Pat. No. 3,373,437, which issued to Sweet et al. in 1967, discloses an array of continuous ink jet nozzles wherein ink drops to be printed are selectively charged and deflected towards the printing plate. This technique is known as binary deflection continuous ink jet, and is used by several manufacturers, including Elmjet and Scitex.
U.S. Pat. No. 3,416,153, which issued to Hertz et al. in 1966, discloses a method of achieving variable optical density of printed spots in continuous ink jet printing using the electrostatic dispersion of a charged drop stream to modulate the number of droplets which pass through a small aperture. This technique is used in ink jet printers manufactured by Iris.
U.S. Pat. No. 3,878,519, which issued to Eaton in 1974, discloses a method and apparatus for synchronizing droplet formation in a liquid stream using electrostatic deflection by a charging tunnel and deflection plates.
U.S. Pat. No. 4,346,387, which issued to Hertz in 1982 discloses a method and apparatus for controlling the electric charge on droplets formed by the breaking up of a pressurized liquid stream at a drop formation point located within the electric field having an electric potential gradient. Drop formation is effected at a point in the field corresponding to the desired predetermined charge to be placed on the droplets at the point of their formation. In addition to charging tunnels, deflection plates are used to actually deflect drops.
Conventional continuous ink jet utilizes electrostatic charging tunnels that are placed close to the point where the drops are formed in a stream. In this manner individual drops may be charged. The charged drops may be deflected downstream by the presence of deflector plates that have a large potential difference between them. A gutter (sometimes referred to as a "catcher") may be used to intercept the charged drops, while the uncharged drops are free to strike the printing plate. In the current invention, the electrostatic charging tunnels are unnecessary.
It would be desirable to have an jet drop printing process to prepare graphic arts media of all kinds economically, and with high speed, without the limitations of having to use electrically conductive liquids.